1. Field Of Invention
This invention relates to an apparatus for molding plastic objects. More specifically, this invention relates to an apparatus for molding plastic parts, in which the means for ejecting the finished part from a cavity mold are positioned on the same side of the mold as the parts for injecting liquid plastic. This allows one side of the finished plastic part to remain completely unblemished by the molding process.
2. Description of the Prior Art
It is common to mold parts for plastic objects (e.g. plastic containers and mini-disk cartridge shells) using an apparatus incorporating a number of metal plates. As shown in FIGS. 10-12, such an apparatus may include a core plate 30', a cavity plate 40' and an ejector plate 20'. Carved out areas in the interiors of the core plate and the cavity plate form the cavity of the mold when these are mated. The ejector plate is usually positioned behind the cavity plate. A plurality of ejector pins 22' extend outwardly from the ejector plate and pass through holes in the cavity plate. Thus when the ejector plate is moved forward the ejection pins are urged against any newly molded part in the cavity plate, thereby forcing the part out of the cavity plate. Thus, when the ejector plate is moved forward, the ejector pins are urged against any newly molded part in the cavity plate thereby forcing the part out of the cavity plate.
Typically, plastic is injected from a line 70' coming from an injection unit into the mold through one or more openings in the core plate. The flow of liquid plastic material may be controlled by a valve 60' having a nozzle 62' and a needle 64' within the nozzle. The movement of the needle in to and out of the valve opening controls the flow of liquid plastic into the mold's cavity 52'. Movement of the needle may be controlled, for example, by a small hydraulic or air valve cylinder 66'. When the end 65' of the needle 64' is flush with the tip 67' of the nozzle 62', the valve 60' is closed (See FIG. 7). As shown in FIG. 10, when the cylinder 66' retracts the needle 64' the valve 60' is opened (See FIG. 8) and liquid plastic material is allowed to enter the cavity of the mold 52'. After the cavity 52' has been filled, the valve cylinder 66' pushes the needle 64' forward to the closed position (See FIG. 7), such that the end 65' of the needle 64' is again flush with the tip 67' of the nozzle 62'.
A prior art method of removing finished parts from a mold is shown in FIGS. 11 and 12. After the plastic material has partially cooled, the cavity plate 40' and the ejector plate 20' both move in tandem away from the core plate 30', which remains stationary. This causes the molded plastic part 50' to be removed from the core plate 30'. Finally, as shown in FIG. 12, a hydraulic or air cylinder 24' pushes the ejector plate 20' forward such that all of the ejector pins 22' simultaneously dislodge the molded plastic object 50' from the cavity plate 40'. This sequence of events may be repeated for the next plastic object 50'. Unfortunately, the movement of the ejector pins against the partially cooled plastic part usually results in a blemish being produced at the point of contact. This blemish is on the part's opposite side, which has received a blemish produced by the injector nozzle.
Most plastic parts have an appearance side, usually the front side, in which a label or some other form of identification is placed. In present molding processes, as described above, blemishes are left on this appearance side by either the valve or the ejector pins. Manufacturers of these parts have, therefore, typically placed paper labels over the appearance side surface as a means of identifying the object and as a means of covering the blemishes. However, due to the increased use of silk screening on plastic objects, a need exists for a plastic object with an appearance side surface which remains unblemished during the molding process such that, for example, a substantial portion of the appearance side surface may be silk screened. Though silk screening is a significant reason for needing an unblemished surface, this is only one of many other purposes in which an unmarked plastic surface may be utilized.
A problem which has prevented present molds from being constructed with injection valves on the same side of the mold as the ejector pins is that ejector pins must be evenly distributed across the entire surface of the finished plastic part so as to apply evenly distributed pressure across an entire surface of the part. This is necessary to prevent warpage of the part upon removal from the mold. However, because the valve nozzle or nozzles often occupy a substantial amount of space, there has previously been no device or method for positioning the ejector pins on the same side of the mold as the valve nozzle which allows proper spacing of the pins to prevent warpage of the plastic part upon its removal from the mold's cavity.